Phase-Controlled Electronic and Structural Tunability in Cu–Se Heterostructures

Prakash Gautam^1*, Lakmal Ruwan Kumara¹, Cheng-Yu Kuo¹, Liang-Wei Lan¹, Chein-Cheng Kuo¹

¹Department of Physics, National Sun Yat-sen University, Kaohsiung, Taiwan

* Presenter:Prakash Gautam, email:om.gautam49501@gmail.com

Copper selenides are an attractive class of materials in plasmonics, energy conversion, superionic conductivity, and superconductivity applications due to their different phase compositions and stoichiometry-dependent electronic tunability [1, 2]. However, a comprehensive understanding of how synthesis conditions determine the atomic-scale structure of these phases on a single-crystal surface and how that structure determines their local electronic properties has not been fully understood. To address this, we use scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) to probe the structural and electronic phase evolution of the Cu-Se phases on Cu(111), synthesized through direct selenization. We investigate the three different Cu-Se phases: defect-void CuSe, 1D strip-patterned CuSe, and a Cu-Se phase with a hexagonal moire pattern. Under specific growth conditions, we observe the coexistence of defect-void CuSe and hexagonal moire-patterned phases. The 1D strip-patterned CuSe emerges at low Se deposition, followed sequentially by the formation of defect-void CuSe, a coexisting phase, and finally the hexagonal moire-patterned Cu-Se phase at the deposition times of 30, 60, 100, and 120 seconds, respectively. Field emission resonance (FER) spectroscopy and local density of states (LDOS) mapping show unique electronic signatures in each phase. The defect-void CuSe has two surface state peaks at 1.10 V and 1.90 V, whereas the 1D strip-patterned CuSe phase has one at 1.40 V. The hexagonal moire-patterned phase, on the other hand, does not show any surface state peak. Moreover, the electronic band gaps are different: the hexagonal moire-patterned phase has a band gap of 0.82 eV at room temperature and 1.10 eV at liquid nitrogen temperature, whereas 1D strip-patterned CuSe and defect-void CuSe have comparatively smaller band gaps. Furthermore, FER analysis shows a local work function variation between 3.73 eV and 4.17 eV across the observed phases, highlighting electronic tunability linked to the structural differences.
By combining LEED and a structural simulation using Python, we quantify the structural variations observed in the STM images and multiple rotational alignments with a ~ 5° offset. Together, these integrated results establish a direct link between growth conditions, atomic arrangement, and local electronic properties. These insights provide a major foundation for phase-controlled engineering of Cu-Se heterostructures with tunable structural and electronic properties, paving the way for their use in nanoscale device applications.

References
[1] E. M. Williamson, Z. Sun, B. A. Tappan, and R. L. Brutchey, J. Am. Chem. Soc. 145, 17954 (2023).
[2] R. Matsumoto, S. Yamamoto, S. Adachi, H. Tanaka, T. Shinmei, T. Irifune, and Y. Takano, Eur. Phys. J. B 98, 154 (2025)

Keywords: Field emission resonance, Local density of states, Local work function, Rotational alignment